This invention relates to a modulating/demodulating circuit in a magnetic recording/playback system of the type in which a frequency-modulated still video signal is recorded on a rotating magnetic disk by a magnetic head or a still video signal is read from the magnetic disk and played back, wherein the modulating/demodulating circuit is used to superimpose data other than a still video signal on a video signal and write the data onto the magnetic disk, or read the data from the magnetic disk, by means of frequency multiplexing.
Electronic still video camera systems have recently been developed. These systems combine an imaging device such as a solid state imaging element or image pickup tube with a recorder that employs an inexpensive magnetic disk of a comparatively large storage capacity as a storage medium and operates by electronically imaging a subject, recording a still picture of the subject on the magnetic disk and reproducing the recorded picture by a separately provided television system or printer. A still video signal magnetic recording system in which a still picture recorded on a visible recording medium such as ordinary film or photographic paper is imaged and recorded on a magnetic disk has also been realized.
Recording a still video signal on a magnetic disk is performed by frequency modulating the video signal over a wide frequency range on the order of 10 MHz and using a comparatively high frequency. It is possible to avoid utilizing certain comparatively low frequencies (e.g. on the order of 200 kHz) to allow their use for the sake of recording data other than video signals. In accordance with the format standards used in this industry, the recording of various data on magnetic disks by frequency multiplexing using such low frequencies is allowed.
An example of a modulating method which can be used to record such data is differential phase shift keying (DPSK), the details of which will be described below. An example of a known modulating/demodulating circuit which operates based on this modulating method is a circuit using an analog-type phase lock loop (PLL), as disclosed, for example, in the specification of Japanese Utility Model Application Laid-Open Publication No. 59-164918. A problem with this conventional modulating/demodulating circuit is that since analog means are used, it is difficult to determine and adjust the circuit constants. Another problem stemming from the analog approach is that the circuitry is of a comparatively complicated construction.